The government today will announce the deployment of British commandos and special forces to the military campaign in Afghanistan amid a growing awareness in London and Washington that the war will be tough, protracted and dangerous.

Tony Blair has warned of British casualties in operations which Admiral Sir Michael Boyce, chief of the defence staff, has said could last throughout the winter and into next spring.

Today's announcement to MPs by the armed forces minister, Adam Ingram, comes after weeks of joint US-British planning, mostly at the US Central Command headquarters in Tampa, Florida. Britain has so far played a largely token role in the military operations, launching a handful of Tomahawk cruise missiles at Afghanistan and providing support aircraft in a gesture of solidarity.

With the onset of the land war, Britain is expected to deploy about 1,000 marines, over 100 SAS and Special Boat Service troops to carry out hit-and-run raids in Afghanistan, and set up temporary bridgeheads in areas of the country regarded as safe from attack by Taliban militia and Osama Bin Laden's al-Qaida forces.

The scale of the problem awaiting the British forces was underlined by a senior US navy officer who said the Taliban was proving a much tougher opponent than expected, and by Donald Rumsfeld, the defence secretary, who admitted he did not know whether Osama bin Laden would ever be caught.

According to a transcript issued by the White House after some controversy over the precise words used by the defence secretary, Mr Rumsfeld tried to shift the public focus away from a preoccupation with capturing or killing the Saudi fugitive. "Well, it's a very difficult thing to do. It's a big world. There are lots of countries. He's got a lot of money. He's got a lot of people who support him. I just don't know whether we'll be successful," Mr Rumsfeld told the USA Today newspaper.

Nineteen days into Operation Enduring Freedom, US pilots continue to bomb Taliban targets, but have not succeeded in dislodging the militia's leadership, nor in flushing out Bin Laden and his al-Qaida organisation. A Pentagon spokesman, Rear Admiral John Stufflebeem, conceded that he was "a bit surprised at how doggedly [Taliban forces] are hanging on to their power".

The British commander for the operation, Admiral Sir Michael Boyce, said that in order to hunt down Bin Laden and his supporters, special forces might have to be sent into Afghanistan for weeks at a time.